PROJECT SUMMARY/ABSTRACT Although significant advances have been made in elucidating the cellular, biophysical and molecular mechanisms of learning and memory, much less is known about the ways in which mnemonic processes are embedded in neuronal networks, thereby storing and expressing a memory via changes in neural activity. The overall goal of this proposal is to provide insights into the design principles that govern the implementation of memories within the complex environment of a neural circuit. Studies will focus on an established in vitro analogue of operant conditioning (OC) in a relatively complex neural circuit, which is amenable to cellular and biophysical analyses. A combination of intracellular electrophysiological recording techniques and voltage- sensitive dye (VSD) recordings will locate and analyze loci of non-synaptic plasticity and synaptic plasticity. In addition, the project will examine the extent to which short- and long-term memory share plasticity loci. Aim 1 will use intracellular recording techniques to examine loci of OC-induced plasticity. Previous studies of OC in this model system focused primarily on non-synaptic plasticity mechanisms. However, preliminary data indicate that OC also modifies the strength of several synaptic connections in the network. Therefore, Aim 1 will examine OC-induced synaptic and non-synaptic plasticity. Aim 2 will use a combination of intracellular and VSD recordings to identify additional sites of OC-induced plasticity. To date, published studies have examined only five of the ~100 neurons and none of the hundreds of synaptic connections that comprise the neural circuit. To address this shortcoming, large-scale VSD recordings, in combination with intracellular recordings, will be used to identify OC-induced changes in activity and synaptic properties in a substantial proportion of the neurons in the circuit. Aim 3 will determine the extent to which short- and long-term memory share common loci and plasticity mechanisms. Our previous studies indicate that at least one locus of plasticity is common to both short- and long-term memory. Thus, an important question in memory research is to determine the extent to which sites for short-term memory are also sites for long-term memory, or conversely, which sites of plasticity may be unique to long-term memory. By examining these three aims, the project will provide insights into the ways in which the many components of a nervous system orchestrate learning and generate behavior.